1. Field of the Invention
The invention relates to the field of the detection of defects in containers, particularly dents in metal containers.
2. Prior Art
Prior to the filling of containers such as steel and aluminum cans, it is desirable to know if the cans are dented or otherwise deformed. Defective containers, if filled and delivered to retail outlets, are often returned by retailers since consumers find them unacceptable. Moreover, severe dents or deformation in the containers can result in their failure when they are filled and sealed.
There are a number of difficult problems associated with the acoustic examination of containers. In a typical container manufacturing or container filling environment, empty or substantially empty containers are moved along a conveyor. The metal containers, because of this movement, cause a great deal of noise. This noise makes it difficult to acoustically examine the containers in the environment where such examination is most expedient. Thus consideration must be given to this background noise for the reliable detection of defects. Also, when containers are moved on a conveyor they are not perfectly aligned and therefore the position of a container under test varies from container to container. When these containers are acoustically examined the amount of energy imparted to a container when it is struck becomes important. Mechanical alignment of these containers or mechanical position detectors are complicated and costly. Means are required to compensate for this positioning problem.
As will be seen, the present invention provides an apparatus for acoustically detecting defects in containers. The problems associated with a noisy environment and the fact that the precise position of a container is not known are solved.
The closest prior art known to Applicant is described in U.S. Pat. No. 3,802,252. The apparatus described in this patent detects the internal pressure of a sealed container. This information is used to determine overpressurization in aerosol cans, volume of liquid sealed within a can, and also aids in detecting spoilage of food products sealed in containers. The containers are magnetically struck and their frequency response examined since the acoustic "ping" is a function of internal pressure. This patent does not describe any apparatus for detecting dents or deformation in open containers.